This invention relates to position indicating in general and more particularly to improved apparatus for remote position indication in a hydraulic positioning drive with a physically separated work cylinder.
Hydraulic positioning drives of this type, in which the pressure supply system and the hydraulic control devices are separated from the work cylinder moving the positioner, are used where the positioner must be operated in a location exposed to vibration, dirt or temperature or in a confined space. This is the case, for instance, in an extrusion casting machine, where the plug of the pouring ladle is the positioner.
Since in customary double acting work cylinders a certain amount of leakage cannot be avoided, it is necessary to provide a remote indication of the position of the piston in the work cylinder to the positioning drive or to a control integrated into, or controlling, the latter. This remote indication can be accomplished mchanically, e.g., via Bowden cables, or electrically by means of a potentiometer tap actuated by the piston rod of the work cylinder. Both possibilities have considerable disadvantages in practice. The length of the mechanical transmission is limited and is subject to damage. With the electrical position indication, contact problems in the potentiometers must be expected over a period of time; if the positioning cylinder is in an exposed location, a potentiometer cannot always be mounted mechanically in an operationally safe manner.
Thus, there is a need to provide a better arrangement to obtain a remote position for a hydraulic positioning drive with a physically separated work cylinder which is operable even under difficult conditions and does not have the above mentioned shortcomings.